Blow-out preventers are provided in well casings of oil and gas wells as standard practice. Commonly such preventers comprise an integral part of the well casing, being installed as a linear section of the casing with oppositely disposed hydraulically powered rams actuatable either to close an annulus between the casing well and drill pipe or the like centrally disposed within the casing bore or to shear such piping and completely seal the casing. If such equipment fails to function adequately or if casing rupture occurs below the location of of a blow-out preventer, as by a floating drilling platform being shifted from anchored position by heavy seas and moving the casing laterally or for whatever reason, it may be necessary to cut and cap a well casing anywhere along its length.